Chelsea: Diego Costa hamstring concern for Jose Mourinho
Chelsea boss Jose Mourinho suggested Diego Costa is not ready to start more than one game a week after using him as a substitute in the draw with Schalke.
The 25-year-old, who has scored seven goals this season, injured a hamstring playing for Spain on 4 September.
Mourinho said: "If he has one week to let the muscle recover he can start."
Brazil-born Costa passed a late fitness test to play against Swansea in the Premier League on Saturday, scoring his first hat-trick in English football in a 4-2 win.
But Mourinho opted to give striker Didier Drogba his first start since rejoining Chelsea in the summer, for their opening European game of the season.
The 36-year-old Ivorian struggled to make an impact as Cesc Fabregas's 11th-minute opener for the Blues was cancelled out by Klaas-Jan Huntelaar just after the hour mark.
Chelsea, who are top of the Premier League after taking 12 points from a possible 12, travel to Manchester City on Sunday.
"It was not to protect Costa for the game on Sunday, it was because he was not in condition to start the game," added Mourinho.
"Playing on Saturday, three days is not enough for him to recover."
Drogba's appearance was his first for Chelsea in the Champions League since scoring the decisive penalty in the 2012 final against Bayern Munich.
However, he failed to make the most of an opportunity to put Chelsea 2-0 up when he was put through by Eden Hazard, dragging his shot just wide.
"I'm not disappointed with the striker because of one inch, he scores a goal or doesn't score a goal," said Mourinho.
"I am happy with what he did. If he scores that goal in this moment, we're speaking about 2-0, not 1-1."
After Huntelaar's equaliser, Chelsea were the stronger side but found the Schalke defence in inspired form, with substitute Loic Remy seeing a shot cleared off the line.
"We had chances to win the game, but that's football, that's Champions League, that's normal," said Mourinho.
"Congratulations to Schalke because they got their objective. There is a little bit of frustration for us, because we did everything to win and we could not."